Fiber structures are often used as cell growth matrices in the field of regenerative medicine. Investigation of fiber structures include the use of polyglycolic acid employed in surgical sutures and the like (for example, see Non-patent document 1). However, because the fiber structures obtained by such ordinary methods have excessively large fiber sizes, the cell-adhering area is insufficient and fiber structures with smaller fiber sizes have therefore been desired for increased surface area.
On the other hand, methods for producing fiber structures with small fiber sizes include the publicly known electrospinning method (for example, see Patent documents 1 and 2). The electrospinning method comprises a step of introducing a liquid, such as a solution containing a fiber-forming substance, into an electrical field and attracting the liquid toward an electrode to form a fiber substance. Normally, the fiber-forming substance hardens while being attracted from the solution. The hardening is accomplished by, for example, cooling (when the spinning liquid is a solid at room temperature, for example), chemical hardening (treatment with hardening vapor, for example), or solvent evaporation. The obtained fiber substance is captured on an appropriately situated acceptor and may be released therefrom if necessary. The electrospinning method can also directly produce a fiber substance in nonwoven fabric form, and is therefore convenient, requiring no further formation of a fiber structure after reeling.
The use of fiber structures obtained by the electrospinning method as matrices for cell culturing is publicly known. For example, formation of a fiber structure composed of polylactic acid by the electrospinning method, and regeneration of blood vessels by culturing of smooth muscle cells thereon, has been investigated (for example, see Non-patent document 2). However, fiber structures obtained using the electrospinning method tend to be dense structures with short distances between fibers, or in other words, structures with large apparent densities. When such a structure is used as a matrix (scaffold) for cell culturing, the cultured cells accumulate on the surface of each fiber forming the fiber structure as culturing proceeds, forming a thick covering on the fiber surfaces. As a result, it is difficult for solutions containing nutrients and the like to adequately migrate into the fiber structure, such that cell culturing has only been possible near the surfaces of the cells which have been cultured and accumulated on the fiber.    [Patent document 1] Japanese Unexamined Patent Publication SHO No. 63-145465    [Patent document 2] Japanese Unexamined Patent Publication No. 2002-249966    [Non-patent document 1] Ono, F., translated by Aizawa, M., “Saisei Igaku [Regenerative Medicine]”, NTS Publications, Jan. 31, 2002, p. 258.    [Non-patent document 2] Joel D. Stitzel, Kristin J. Pawlowski, Gary E. Wnek, David G. Simpson, Gary L. Bowlin, Journal of Biomaterials Applications 2001, Vol. 16 (U.S.), pp. 22-33.